Stainless steels are alloys of iron with chromium, nickel and other expensive metals and are known to be resistant to attack by many types of corrosive influences and it is possible to cast or forge valve bodies of sufficient thickness that they can resist such attack for long periods of time, but this adds to the cost of the valve and the increase in weight is often unacceptable for the intended use.
Futhermore many stainless steel alloys are formulated with constituents such as sulfur added to make them readily fabricable, e.g. by machining, and many such alloys are known to be subject to pitting corrosion when exposed to specific environments such as those containing HCl, HF, H.sub.2 S and other acidic gases.
One proposed means for increasing the service life of metal valves has been to provide a lining of corrosion resistant material for those surfaces exposed to the corrosive medium. Such linings are known in U.S. Pat. No. to Newton U.S. Pat. No. 2,352,799; Vogler U.S. Pat. No. 3,120,247; Fitting U.S. Pat. No. 3,568,709; Becker U.S. Pat. No. 4,356,612 and Kawai U.S. Pat. No. 4,601,087 and others of a similar nature. The constructions shown in these patents offer little or no protection against corrodents which leak from the fittings adjacent the valve body and which then enter the atmosphere around the valve, and some of which may ultimately condense on the exterior surfaces of the valve which is formed of an alloy susceptible to corrosive attack by the condensate.
Other previously suggested means for protecting alloy steel valve bodies from corrosive attack include constructing a housing or enclosure around the valve body as shown in Rentschler U.S. Pat. No. 3,367,360 issued Feb. 6, 1968; casting a composite as shown in Waltien U.S. Pat. No. 3,446,236 issued May 27, 1969; or encapsulating the valve in a sheath of elastic distendable synthetic resin as shown in David U.S. Pat. No. 3,738,383 issued June 12, 1973 and Peroux U.S. Pat. No. 4,510,965 issued Apr. 16, 1985. Such proposals require expensive apparatus to carry out the procedures described and have not been widely adopted in the field.